Reduced differential sticking drilling collar

ABSTRACT

Systems and methods for drilling a subterranean well include a reduced differential sticking drilling collar. The reduced differential sticking drilling collar is an elongated tubular member having a minimum outer diameter at an uphole end and at a downhole end. A collar body is located between the uphole end and the downhole end. The collar body has a cross section of alternating lobes and troughs. The collar body has a maximum outer diameter that is larger than the minimum outer diameter. A taper section extends from the minimum outer diameter to the maximum outer diameter. The lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 63/066,791, filed Aug. 17, 2020, titled “Low Contact Area Drilling Collar,” the full disclosure of which is hereby incorporated herein by reference in its entirety for all purposes.

BACKGROUND OF THE DISCLOSURE 1. Field of the Disclosure

The present disclosure relates to subterranean well development, and more specifically, the disclosure relates to collars that are used in drill strings during the drilling of the subterranean well.

2. Description of the Related Art

During the drilling of a wellbore of a subterranean well, a drilling collar can be added to the drill string to add additional weight to the drilling bit. A drilling collar is large diameter, heavy walled drill pipe. In general the more weight that can be applied, the faster the rate of penetration (ROP), unless other drilling dysfunctions prevent a faster ROP, such as shock and vibration or pipe buckling.

The drilling collar can be part of the bottom hole assembly, which is commonly positioned adjacent or proximate to the drilling bit. Currently available drilling collars can be cylindrical in shape. Alternate currently available drilling collars can be cylindrical in shape with a spiraled profile with shallow flutes machined into an outer diameter surface of the drilling collar. Drilling collars are commonly formed of steel or alternately a non-metallic variant of steel. Drilling collars can have central bores to allow for the passage of drilling fluid through the central bore of the drilling collar.

SUMMARY OF THE DISCLOSURE

During drilling operations there is a risk of the drill string becoming stuck in the wellbore. As an example, mechanical interference or differential pressures can cause a stuck pipe. Differential pressure pipe sticking can occur when there is an overbalance of pressure in the hole compared to the formation and the pipe becomes embedded in a mudcake along a wall of the wellbore. The risk of a pipe getting stuck increases with the larger the overbalance of pressure, the more surface area of the pipe that is in contact with the wall of the wellbore, the vector sum of combined forces, and the longer the time that the pipe spends stationary.

Drilling long horizontal well sections can be especially problematic. Cuttings may settle and accumulate, particularly on the low side of the wellbore. Cuttings can continue to accumulate until the reduced flow area attains a velocity sufficient to carrying cuttings again. When this happens the particles are picked up by flow at an equilibrium layer.

In currently available drilling collars that are full hole collars with a cylindrical or square profile, such drilling collars are unchanging along an entire axial length. Therefore full surface contact is expected along the full length of the tool in a single plane. The drilling collar according to embodiments of this application instead is spiraled along an axial length so that the contact point moves up the tool with the spiral so no one plane is preferentially selected. On rotation, the area of reduced differential sticking drilling collar that is in contact with the formation moves axially. The reduced differential sticking drilling collar is also nearer the hole size and the pressure exerted by the mud on the tool is more evenly distributed and more balanced in every direction.

In embodiments of this disclosure, the vector summed forces due to well pressure of the reduced differential sticking drilling collar is decreased compared to currently available drilling collars. Systems and methods of this disclosure also provide a stiffer, heavier collar that maximizes collar bending stiffness. Due to the shape of the drilling collar in accordance with embodiments of this disclosure, there is sufficient bypass area and a flow path past the drilling collar that allows for the evacuation of cuttings from the wellbore. In addition, the external profile of the drilling collar of embodiments of this disclosure can agitate cuttings and can prevent the cuttings equilibrium layer from forming around the collar.

In an embodiment of this disclosure, a system for drilling a subterranean well includes a reduced differential sticking drilling collar. The reduced differential sticking drilling collar is an elongated tubular member having a minimum outer diameter at an uphole end and at a downhole end. A collar body is located between the uphole end and the downhole end. The collar body has a cross section of alternating lobes and troughs. The collar body has a maximum outer diameter that is larger than the minimum outer diameter. A taper section extends from the minimum outer diameter to the maximum outer diameter. The lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern.

In alternate embodiments, the taper section can include reaming elements, the reaming elements extending radially outward from the taper section. The taper section can alternately include cutting elements, the cutting elements extending obliquely radially outward from the taper section. An angle of the taper section relative to a central axis of the reduced differential sticking drilling collar can be in a range of 10 to 30 degrees. In alternate embodiments, the angle of the taper section relative to the central axis of the reduced differential sticking drilling collar can be less than 10 degrees or greater than 30 degrees. A number of lobes of a cross section of the collar body can be at least three. In alternate embodiments, the number of lobes of a cross section of the collar body can be two. The lobes of the collar body can wind around no more than one half of a circumference of the collar body over an entire axial length of the lobes. The lobes of the collar body and the troughs of the collar body can be curved in shape. An outermost surface of the lobes can be parallel to a central axis of the reduced differential sticking drilling collar.

In an alternate embodiment of this disclosure, a system for drilling a subterranean well includes a rotating drilling string formed of a plurality of tubular drill string joints. Each of the plurality of tubular drill string joints has a joint outer diameter. A reduced differential sticking drilling collar is secured in-line with the plurality of tubular drill string joints. The reduced differential sticking drilling collar has a minimum outer diameter at an uphole end and at a downhole end. An uphole connector of the reduced differential sticking drilling collar is operable to secure the uphole end to an uphole adjacent drill string member. A downhole connector of the reduced differential sticking drilling collar is operable to secure the downhole end to a downhole adjacent drill string member. A collar body is located between the uphole end and the downhole end. The collar body has a cross section of alternating lobes and troughs. The collar body has a maximum outer diameter that is larger than the minimum outer diameter and larger than the joint outer diameter. The lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern.

In alternate embodiments, a bypass area can be defined between an outer surface of the collar body and an inner diameter surface of a wellbore of the subterranean well. A taper section can extend from the minimum outer diameter to the maximum outer diameter. An angle of the taper section relative to a central axis of the reduced differential sticking drilling collar can be in a range of 10 to 30 degrees. In alternate embodiments, the angle of the taper section relative to the central axis of the reduced differential sticking drilling collar can be less than 10 degrees or greater than 30 degrees. The lobes of the collar body can wind around no more than one half of a circumference of the collar body over an entire axial length of the lobes.

In another alternate embodiment of this disclosure, a method for drilling a subterranean well includes providing a reduced differential sticking drilling collar. The reduced differential sticking drilling collar is an elongated tubular member having a minimum outer diameter at an uphole end and at a downhole end. A collar body is located between the uphole end and the downhole end. The collar body has a cross section of alternating lobes and troughs. The collar body has a maximum outer diameter that is larger than the minimum outer diameter. A taper section extends from the minimum outer diameter to the maximum outer diameter. The lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern. The reduced differential sticking drilling collar is secured in line with a plurality of tubular drill string joints to form a drilling string. The drilling string is rotated to form the subterranean well.

In alternate embodiments, the taper section can include reaming elements extending radially outward from the taper section, and the method can further include reaming within a wellbore of the subterranean well with the reaming elements. The taper section can alternately include cutting elements extending obliquely radially outward from the taper section, and the method can further include cutting within a wellbore of the subterranean well with the cutting elements.

In other alternate embodiments, an angle of the taper section relative to a central axis of the reduced differential sticking drilling collar can be in a range of 10 to 30 degrees. In alternate embodiments, the angle of the taper section relative to the central axis of the reduced differential sticking drilling collar can be less than 10 degrees or greater than 30 degrees. A number of lobes of a cross section of the collar body can be at least three. In alternate embodiments, the number of lobes of a cross section of the collar body can be two. The lobes of the collar body can wind around no more than one half of a circumference of the collar body over an entire axial length of the lobes. The lobes of the collar body and the troughs of the collar body can be curved in shape. An outermost surface of the lobes can be parallel to a central axis of the reduced differential stickingdrilling collar.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, aspects and advantages of the embodiments of this disclosure, as well as others that will become apparent, are attained and can be understood in detail, a more particular description of the disclosure may be had by reference to the embodiments thereof that are illustrated in the drawings that form a part of this specification. It is to be noted, however, that the appended drawings illustrate only certain embodiments of the disclosure and are, therefore, not to be considered limiting of the disclosure's scope, for the disclosure may admit to other equally effective embodiments.

FIG. 1 is a schematic section view of a subterranean well having a reduced differential sticking drilling collar, in accordance with an embodiment of this disclosure.

FIG. 2 is front view of a reduced differential sticking drilling collar, in accordance with an embodiment of this disclosure.

FIG. 3 is detailed front view of an uphole end of a reduced differential sticking drilling collar, in accordance with an embodiment of this disclosure.

FIG. 4 is a perspective section view of a reduced differential stickingdrilling collar, in accordance with an embodiment of this disclosure.

FIG. 5 is a perspective section view of an alternate reduced differential sticking drilling collar, in accordance with an embodiment of this disclosure.

FIG. 6 is a section view of a prior art conventional collar located within a subterranean well.

FIG. 7 is a section view of a reduced differential sticking drilling collar located within a subterranean well, in accordance with an embodiment of this disclosure.

FIG. 8 is a chart showing results of performance calculation results for drilling collars by wellbore size and filter cake thickness.

FIG. 9A-9B is a detailed front view and a cross section view of a prior art cylindrical drilling collar.

FIG. 10A-10B is a detailed front view and a cross section view of a prior art spiral fluted drilling collar.

FIG. 11A-11B is a detailed front view and a cross section view of a reduced differential sticking drilling collar, in accordance with an embodiment of this disclosure.

FIG. 12A-12C are section views of a reduced differential sticking drilling collar shown at 0°, 45° and 90° of rotation within a wellbore, in accordance with an embodiment of this disclosure.

FIG. 13A-13C are elevation views of wellbore wall contact of a reduced differential sticking drilling collar shown at 0°, 45° and 90° of rotation within a wellbore, in accordance with an embodiment of this disclosure.

FIG. 14A-14C are section views of a prior art cylindrical drilling collar shown at 0°, 45° and 90° of rotation within a wellbore, in accordance with an embodiment of this disclosure.

FIG. 15A-15C are elevation views of wellbore wall contact of a prior art cylindrical drilling collar shown at 0°, 45° and 90° of rotation within a wellbore, in accordance with an embodiment of this disclosure.

FIG. 16A-16C are section views of a prior art spiral fluted drilling collar shown at 0°, 45° and 90° of rotation within a wellbore, in accordance with an embodiment of this disclosure.

FIG. 17A-17C are elevation views of wellbore wall contact of a prior art spiral fluted drilling collar shown at 0°, 45° and 90° of rotation within a wellbore, in accordance with an embodiment of this disclosure.

DETAILED DESCRIPTION

The disclosure refers to particular features, including process or method steps. Those of skill in the art understand that the disclosure is not limited to or by the description of embodiments given in the specification. The subject matter of this disclosure is not restricted except only in the spirit of the specification and appended Claims.

Those of skill in the art also understand that the terminology used for describing particular embodiments does not limit the scope or breadth of the embodiments of the disclosure. In interpreting the specification and appended Claims, all terms should be interpreted in the broadest possible manner consistent with the context of each term. All technical and scientific terms used in the specification and appended Claims have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs unless defined otherwise.

As used in the Specification and appended Claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly indicates otherwise.

As used, the words “comprise,” “has,” “includes”, and all other grammatical variations are each intended to have an open, non-limiting meaning that does not exclude additional elements, components or steps. Embodiments of the present disclosure may suitably “comprise”, “consist” or “consist essentially of” the limiting features disclosed, and may be practiced in the absence of a limiting feature not disclosed. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

Where a range of values is provided in the Specification or in the appended Claims, it is understood that the interval encompasses each intervening value between the upper limit and the lower limit as well as the upper limit and the lower limit. The disclosure encompasses and bounds smaller ranges of the interval subject to any specific exclusion provided.

As used in this Specification, the term “substantially equal” means that the values being referenced have a difference of no more than two percent of the larger of the values being referenced.

Where reference is made in the specification and appended Claims to a method comprising two or more defined steps, the defined steps can be carried out in any order or simultaneously except where the context excludes that possibility.

Looking at FIG. 1, subterranean well 10 extends downwards from a surface of the earth, which can be a ground level surface or a subsea surface. Wellbore 12 of subterranean well 10 can extended generally vertically relative to the surface. Wellbore 12 can alternately include portions that extend generally horizontally or in other directions that deviate from generally vertically from the surface. Subterranean well 10 can be a well associated with hydrocarbon development operations, such as a hydrocarbon production well, an injection well, or a water well.

Drilling string 14 extends into wellbore 12 of subterranean well 10. Drilling string 14 can include downhole tools and equipment that are secured in line along drilling string 14. Drilling string 14 can have, for example, a bottom hole assembly 16 that can include a drill bit 18. Drill bit 18 can rotate to create wellbore 12 of subterranean well 10. Bottom hole assembly 16 can further include other tools and equipment.

Drilling string 14 can be formed of a number of tubular drill string joints 20. Drill string joints 20 can be threaded members that are secured to adjacent drill string joints 20. Drill string joint 20 can have an inner bore for delivering drilling fluid from the surface to a location downhole, such as to bottom hole assembly 16. Note that FIG. 1 is schematic in nature and not to scale.

Drilling string 14 can further include reduced differential sticking drilling collar 22. Looking at FIGS. 2-3, reduced differential sticking drilling collar 22 is an elongated member with a central axis Ax, uphole end 24, and downhole end 26. Uphole end 24 and downhole end 26 have a minimum outer diameter 28. Minimum outer diameter 28 can be the same as joint outer diameter 29 (FIG. 1). Joint outer diameter 29 is the outer diameter of drill string joint 20.

Reduced differential sticking drilling collar 22 can include uphole connector 30 at uphole end 24. Uphole connector 30 can secure uphole end 24 of reduced differential sticking drilling collar 22 to an uphole adjacent drill string member. In the example embodiment of FIG. 2, uphole connector 30 is a female threaded connector that can be threaded to an uphole adjacent drill string member.

Reduced differential sticking drilling collar 22 can include downhole connector 32 at downhole end 26. Downhole connector 32 can secure downhole end 26 of reduced differential sticking drilling collar 22 to a downhole adjacent drill string member. In the example embodiment of FIG. 3, downhole connector 32 is a male threaded connector that can be threaded to a downhole adjacent drill string member.

Collar body 34 is located axially between uphole end 24 and downhole end 26. Looking at FIGS. 4-5, in cross section collar body 34 has alternating lobes 36 and troughs 38. A lobe 36 of collar body 34 is a radially outward protrusion from collar body 34. Lobe 36 is sized so that when reduced differential sticking drilling collar 22 is located with wellbore 12, an outermost surface of lobe 36 is in contact with or proximate to an inner diameter surface of wellbore 12. In the example embodiment of FIG. 7, each lobe 36 is in contact with a layer of filter cake 40 that coats an inside surface of wellbore 12. In alternate embodiments, one or more of the lobes 36 can be proximate to filter cake 40, but not in direct contact with filter cake 40.

In embodiments of this disclosure, the cross section of collar body 34 can have at least three lobes 36. In the example embodiment of FIG. 5, there are three lobes 36. In the example embodiment of FIG. 4, there are four lobes 36. In the example embodiment of FIG. 7, there are five lobes 36. In alternate embodiments, there can be more than five lobes 36. Lobes 36 can have an outer surface that is curved in shape. Lobes 36 can have a smooth shape to the outer surface to reduce contact between lobes 36 and the inner diameter surface of wellbore 12 to reduce the friction forces between lobes 36 and the inner diameter surface of wellbore 12.

A trough 38 of collar body 34 is located between each lobe 36 of collar body 34. Looking at FIG. 7, trough 38 is spaced apart from the inner diameter surface of wellbore 12. The area defined between trough 38 and the inner diameter surface of wellbore 12 is a bypass area 42. Bypass area 42 extends the entire length of collar body 34 and provides a flow path for fluids and solids to flow within wellbore 12 past collar body 34. Trough 38 can be curved in shape. Alternately, as shown in the example embodiment of FIG. 5, trough 38 can be relatively flat between adjacent lobes 36. Trough 38 can have a smooth shape to the outer surface to improve the ability of cutting to travel through bypass area 42, to reduce the risk of erosion on collar body 34, reduce the pressure drop along the axial length of collar body 34, and to reduce stresses, such as bending stresses, with rotation of reduced differential sticking drilling collar 22.

Bypass area 42 and the number of spirals of lobes 36 around a circumference of collar body 34 per axial length of collar body 34 will be sized to allow for sufficient space for solids and fluid that travel within wellbore 12 to pass collar body 34 by way of bypass area 42 easily. The flow rate and solid content of fluids and solids that travel within wellbore 12 can dictate the size requirements for bypass area 42. For example, bypass area 42 can be sized so that the velocity of the solids and fluids traveling through wellbore 12 is maintained below a level that would cause erosion or would result in a large pressure drop.

Looking at FIGS. 2-3, lobes 36 of collar body 34 define a maximum outer diameter 44 of reduced differential sticking drilling collar 22. Maximum outer diameter 44 is larger than minimum outer diameter 28 of uphole end 24 and downhole end 26 of reduced differential sticking drilling collar 22. Maximum outer diameter 44 is also larger than the joint outer diameter 29 of drill string joint 20.

Looking at FIG. 2, lobes 36 of collar body 34 wind along an entire axial length of collar body 34 in a spiral pattern, which can also be referred to as a helical pattern. The spiral pattern is a gentle spiral. As used in this disclosure, a gentle spiral is defined as a spiral shape that results from lobes 36 of collar body 34 wind around no more than one half of the circumference of collar body 34 over an entire axial length of lobes 36. The entire axial length of lobes 36 is the total length of collar body 34 over which lobes 36 extend. As an example, if lobes 36 of collar body 34 extend an axial length of thirty feet along collar body 34, then lobes 36 can wind around no more than one half of the circumference of collar body 34 over thirty feet of collar body 34. Having a gentle spiral minimizes the resistance along the path of bypass area 42 as solids and fluids travel axially within wellbore 12 past collar body 34.

In embodiments, lobes 36 may not extend to the terminal end of collar body 34. As an example, an axial length of collar body 34 at each terminal end of collar body 34 can be used for tongs or to otherwise make and break connections. Such terminal ends of collar body 34 can be free of lobes 36. This axial length of collar body 34 that is free of lobes 36 can extend one or more feet at either terminal end of collar body 34. A standard length for collar body 34 can be thirty to thirty-one feet. Therefore, if terminal ends of collar body 34 are free of lobes 36, lobes 36 could extend less than thirty feet and could extend, for example, twenty nine feet or less along collar body 34.

In alternate embodiments it may be advantageous for each lobe 34 to wind 360 degrees, or more than 360 degrees around the outer circumference of collar body 34. As an example, lobe 34 could wind up to 720 degrees around the outer circumference of collar body 34. In alternate embodiments, each lobe 34 may wind less than 360 degrees around the outer circumference of collar body 34. As an example, looking at FIG. 11A, each lobe 34, when viewed in cross section, winds less than 360 degrees around the outer circumference of collar body 34 over the length of lobe 34 along collar body 34.

The combination of the cross sectional area of bypass area 42 and the gentle spiral design allow for solids and fluids to travel axially past collar body 34 relatively unobstructed compared to currently available drilling collars. Solids and fluids within wellbore 12 can travel axially past collar body 34 even if reduced differential sticking drilling collar 22 is not rotating. The spiral also aids agitation and encourages upward flow when the low contact area collar is rotating. When the low contact area collar is static, the spiral helps ensure a minimum fluid velocity is maintained and that suspended cuttings are evenly distributed around the low contact area collar circumference.

Looking at FIG. 3, lobes 36 form a spiral pattern along the length of collar body 34 is such a way that a line drawn axially parallel to the outermost surface of the lobe along a length of the spiral is parallel to central axis Ax of reduced differential sticking drilling collar 22. That is, the outer face of lobe 36 does not twist but instead remains parallel to central axis Ax of reduced differential sticking drilling collar 22. In alternate embodiments, the outer face of lobe 36 can twist.

Reduced differential sticking drilling collar 22 can be manufactured from a single tubular member. The helical pattern of alternating lobes 36 and troughs 38 can be manufactured using, as an example, multi-axis CNC machining, which can allow for cost-effective mass production.

Looking at FIG. 3, collar body 34 includes slot 46. Slot 46 can be used by a tool when making up drilling string 14. As an example, a tool can be used to grasp reduced differential sticking drilling collar 22 so that relative rotation between reduced differential sticking drilling collar 22 can be used to thread reduced differential sticking drilling collar 22 to adjacent members. In certain embodiments, slot 46 can be located a distance of 3 to 5 feet from an end surface of uphole end 24 of reduced differential sticking drilling collar 22.

In order to transition between the minimum outer diameter 28 of uphole end 24 and downhole end 26, and the maximum outer diameter 44 of collar body 34, reduced differential sticking drilling collar 22 further includes taper section 48. Taper section 48 extends from minimum outer diameter 28 to maximum outer diameter 44. Taper section 48 can be sized so that taper angle 50 is in a range of 10 to 30 degrees. In alternate embodiments, taper angle 50 can be less than 10 degrees or greater than 30 degrees. Taper angle 50 is the angle of the taper section relative to central axis Ax of reduced differential sticking drilling collar 22.

Looking at FIG. 3, taper section 48 can include reaming elements 52. Reaming elements 52 can extend radially outward from taper section 48. Reaming elements 52 can be positioned along the outer face of lobe 36 of taper section 48. Reaming elements 52 can be used to ream an interior surface of wellbore 12. Taper section 48 can alternately include cutting elements 54. Cutting elements 54 can extend obliquely radially outward from taper section 48. Cutting elements 54 can be positioned along the outer face of lobe 36 of taper section 48. Cutting elements 54 can be used to cut an interior surface of wellbore 12. In alternate embodiments, taper section can include only reaming elements 52, can include only cutting elements 54, or can include both reaming elements 52 and cutting elements 54. Reaming elements 52 and cutting elements 54 can help to trim wellbore 12 to gauge during drilling or reaming operations.

In embodiments, collar body 34 can have a wear resistant material. The wear resistant material can form an outer layer of lobe 36 only or can cover an entire surface of collar body 34. The wear resistant material can be a layer or a coating and can be, for example, a tungsten carbide, a thermally stable polycrystalline, a diamond, or other available wear resistant material. Alternately, collar body 34 can have a friction reducing material. Some suitable friction reducing materials can be hard and have a low friction coefficient. As an example the friction reducing material can be Hardide, hard chrome or other commercially available friction reducing material.

In an example of operation, to drill a subterranean well reduced differential sticking drilling collar 22 can be secured in-line with a plurality of tubular drill string joints 20 to form drilling string 14. Drilling string 14 can be rotated and lowered through a subterranean formation to form wellbore 12.

Performance Data Differential Sticking

When a drilling collar is freely suspended in a wellbore, the hydrostatic pressure acts evenly on the exterior surface of the drilling collar. In such an arrangement, the hydrostatic pressure causes no net side load to act on the drilling collar.

If the drilling collar becomes embedded into the filter cake the hydrostatic pressure no longer acts on the full exterior of the drilling collar. Instead, the hydrostatic pressure only acts on the remaining exposed outer surfaces of the drilling collar. Meanwhile, a pressure gradient occurs at the drilling collar area that is embedded in the filter cake. This pressure gradient is the difference between the hydrostatic pressure and formation pressure. If the formation pressure is lower than the well hydrostatic pressure the drilling collar will be forced into the formation until the reaction force from the formation opposes and resists further penetration. The drilling string may become stuck when the pressure imbalance exceeds the capacity of the rig equipment to rotate or axially move the drilling string, or exceeds the strength of the drilling string.

Looking at FIGS. 6-7, the performance of a prior art cylindrical drilling collar 22′ as shown in FIG. 6 was compared to a five lobed reduced differential sticking drilling collar 22 of FIG. 7. Looking at FIG. 6, the features that have been described in relation to reduced differential sticking drilling collar 22 have been labeled with the same number in relating to the prior art cylindrical drilling collar and marked with a single prime ′ indicator.

As used in this disclosure the embedded volume 56 is the sum of the total volume of the drilling collar that is embedded in the layer of filter cake 40. As used in this disclosure, projected length 58 is the projected distance measured perpendicular to the wall over which the differential pressure will force the drilling collar against the formation. Embedded area 56 only includes the portion of the drilling collar that is embedded into filter cake 40.

The total force may be determined by applying the appropriate pressures to corresponding areas on the perimeter of the drilling collar. Looking at FIG. 6, the projected contact length 58′ for the cylindrical prior art drilling collar 22′ is determined. This is the length over which the collar is embedded into the layer of filter cake 40′.

When determining the sum of the forces on cylindrical prior art drilling collar 22′, either hand calculations or finite element analysis can be used due the simple shape of the perimeter. Calculation of the sum or the forces on a non-cylindrical collar shapes is more difficult, particularly where the contact position changes along the collar length. For example, when reduced differential sticking drilling collar 22 of FIG. 7 rotates, both the projected contact length 58′ and the embedded volume 56 will change as the spiral shaped lobes 36 of collar body 34 pass through filter cake 40. Therefore the sum or the forces on reduced differential sticking drilling collar 22 have been determined by finite element analysis.

Tables 1-3 and FIG. 9 provide the results of the performance analysis for certain filter cake thickness and wellbore diameter. Such performance analysis was evaluated at a 5,000 pounds per square inch (psi) hydrostatic pressure and a 4,000 psi formation pressure, resulting in a 1,000 psi overbalance.

TABLE 1 Comparison of Drilling Collar Characteristics in 8.5 inch Diameter Wellbore (Hole) Reduced Cylindrical Differential Filter Cake Wellbore Drilling Collar Sticking Thickness Diameter Load Drilling Collar Side Load (in) (in) (lbf/ft) Load (lbf/ft) Reduction 0.05 8.5 30,084 8,709 71% 0.10 8.5 41,640 9,079 78% 0.15 8.5 49,872 5,102 90% 0.20 8.5 56,304 3,816 93% 0.25 8.5 61,500 3,033 95%

TABLE 2 Comparison of Drilling Collar Characteristics in 9.0 inch Diameter Wellbore (Hole) Cylindrical Reduced Drilling Differential Filter Cake Wellbore Collar Sticking Thickness Diameter Load Drilling Collar Side Load (in) (in) (lbf/ft) Load (lbf/ft) Reduction 0.05 9.0 27,396  4,513 84% 0.10 9.0 38,052  8,543 78% 0.15 9.0 45,756 12,088 74% 0.20 9.0 51,852 15,146 71% 0.25 9.0 56,880 17,686 69%

TABLE 3 Comparison of Drilling Collar Characteristics in 9.5 inch Diameter Wellbore (Hole) Cylindrical Reduced Drilling Differential Filter Cake Wellbore Collar Sticking Thickness Diameter Load Drilling Collar Side Load (in) (in) (lbf/ft) Load (lbf/ft) Reduction 0.05 9.5 25,512  3,472 87% 0.10 9.5 35,520  6,701 82% 0.15 9.5 42,816  9,664 78% 0.20 9.5 48,648 12,371 75% 0.25 9.5 53,496 14,824 72%

As can be seen from the example performance calculations, use of reduced differential sticking drilling collar 22 can reduce side loads by between 69% and 95%, compared to the use of prior art cylindrical drilling collar 22′.

Unlike prior art cylindrical drilling collar 22′, low contract area drilling collar 22 may make simultaneous contact with the cake layer at multiple locations around the collar perimeter. Differential pressure generates a side load at each contact area. Looking at FIG. 7, the vector sum of side loads reduces when multiple contact areas are distributed around the collar perimeter; opposing forces are cancelled.

In assessing the performance of low contract area drilling collar 22 in the 8.5 in hole it is noted that lobes 36 embed at multiple locations, particularly when the layer of filter cake 40 is thick. This is beneficial in reducing side load and explains the especially favorable performance for low contract area drilling collar 22 in 8.5 in hole, as can be seen in FIG. 9.

The reduction of the side load of drilling collars is directly proportional to the reduction in contact area between the drilling collar and the wall of the wellbore. As an example, if a prior art cylindrical collar was to have a side force of 30,000 lb/ft and a prior art spiral or fluted collar has 50% reduction in contact area the side load becomes 15,000 lb/ft. The side load experienced by prior art spiral or fluted drilling collars remains high compared to the drilling collar of embodiments of this disclosure.

Wall Contact Area

Looking at FIGS. 9A through 11B, a prior art cylindrical drilling collar 22′ as shown in FIGS. 9A-9B, and a prior art spiral fluted drilling collar 22″ as shown in FIGS. 10A and 10B was compared to a five lobed reduced differential sticking drilling collar 22 as shown in FIGS. 11A and 11B. The results are shown in Tables 4-6. As can be seen from the data in Tables 4-6, reduced differential sticking drilling collar 22 has the benefit of a significantly larger diameter, and the advantage of a lower embedded volume compared to prior art cylindrical drilling collar 22′, and a prior art spiral fluted drilling collar 22″.

As used in this disclosure, the shadow area is the projected length 58 (FIG. 7) multiplied by the length of the drilling collar. As previously noted, the embedded volume is the sum of the total volume of the drilling collar that is embedded in the layer of filter cake 40 (FIG. 7).

TABLE 4 Wall Contact for Prior Art Cylindrical Collar Cake Embedded Collar ‘OD’ Hole ‘HD’ Thickness ‘T’ Shadow Area Volume (in.) (in.) (mm) (in.²/ft.) (in.³/ft.) 6.75 8.5 1 26.817 0.714 6.75 8.5 2 37.289 2.013 6.75 8.5 3 44.890 3.688 6.75 8.5 4 50.933 5.663 6.75 8.5 5 55.936 7.893 6.75 9.0 1 24.402 0.647 6.75 9.0 2 34.025 1.826 6.75 9.0 3 41.08 3.344 6.75 9.0 4 46.751 5.132 6.75 9.0 5 51.505 7.150

TABLE 5 Wall Contact for Prior Art Spiral Fluted Drilling collar Cake Embedded Collar ‘OD’ Hole ‘HD’ Thickness ‘T’ Shadow Area Volume (in.) (in.) (mm) (in.²/ft.) (in.³/ft.) 6.75 8.5 1 21.99 0.584 6.75 8.5 2 30.329 1.646 6.75 8.5 3 36.035 2.998 6.75 8.5 4 40.394 4.569 6.75 8.5 5 43.952 6.321 6.75 9.0 1 20.009 0.530 6.75 9.0 2 27.834 1.496 6.75 9.0 3 33.298 2.731 6.75 9.0 4 37.547 4.172 6.75 9.0 5 42.647 5.782

TABLE 6 Wall Contact for Reduced Differential Sticking Drilling Collar Cake Embedded Collar ‘OD’ Hole ‘HD’ Thickness ‘T’ Shadow Area Volume (in.) (in.) (mm) (in.²/ft.) (in.³/ft.) 8.40 8.5 1 0 0.124 8.40 8.5 2 0 0.606 8.40 8.5 3 0 1.477 8.40 8.5 4 0 2.777 8.40 8.5 5 0 4.332 8.40 9.0 1 2.701 0.101 8.40 9.0 2 3.002 0.247 8.40 9.0 3 3.016 0.397 8.40 9.0 4 3.019 0.546 8.40 9.0 5 3.020 0.761

Progressive Wall Contact Area

Formation porosity is not always uniform around the wellbore, there may be localized areas where porosity is high and areas where porosity is reduced or even absent. These changes may occur around the wellbore perimeter or they may occur longitudinally within the well. Differences in porosity and permeability lead to different cake thicknesses being formed.

Looking at FIGS. 12A-12C, and FIGS. 13A-13C, the outer diameter of reduced differential sticking drilling collar 22 has a helical profile. As reduced differential sticking drilling collar 22 rotates the areas in contact with the formation constantly changes. This may help prevent reduced differential sticking drilling collar 22 becoming stuck as the collar contact areas alternately pass over formation areas of high and low porosity and different cake thicknesses.

A rotating reduced differential sticking drilling collar 22 is continually making and breaking contact with a particular formation area as each helix moves in and out of contact. This may help prevent the Low Stick Drill Collar from becoming differentially stuck.

In contrast, looking at FIGS. 14A-14C, and FIGS. 15A-15C, when a rotating cylindrical prior art drilling collar 22′ is embedded in cake media the collar surface slides over the cake layer with no change in contact area. There is no change in force when cylindrical prior art drilling collar 22′ is rotated over formation areas of high and low porosity.

Looking at FIGS. 16A-16C, and FIGS. 17A-17C, when a rotating prior art spiral fluted drilling collar 22″ is embedded in cake media the collar surface slides over the cake layer with little change in contact area. There is little change in force when prior art spiral fluted drilling collar 22″ is rotated over formation areas of high and low porosity.

There can be additional challenges in drilling long horizontal well sections, particularly in the removal of cuttings. The spiral profile of reduced differential sticking drilling collar 22 acts as an Archimedes screw, continually pushing media axially along the wellbore as the drill collar is rotated. This may aid cuttings removal while drilling horizontally, so further reducing risk of stuck pipe.

Similarly, the spiralled Archimedes screw profile of reduced differential sticking drilling collar 22 may be beneficial when the bottom hole assembly (BHA) is traversing the wellbore transition between horizontal and vertical. Cuttings accumulate in such transitional area, which may impede the BHA. The spiral profile of the reduced differential sticking drilling collar 22 may help to beneficially agitate these cuttings while the BHA moves through the transitional area.

Cuttings within horizontal well sections fall to the low side due to gravity. These cuttings quickly accumulate and restrict the annulus of the wellbore, forming the equilibrium layer. Cylindrical prior art drilling collar 22′ which is embedded in cuttings will act like a plain bearing, rotating in the pocketed recess. The smooth outer diameter surface of cylindrical prior art drilling collar 22′ will slide over the cuttings surface but does not significantly lift cuttings.

Reduced differential sticking drilling collar 22 has lobes 36 which act as paddles. Rotation of reduced differential sticking drilling collar 22 lifts cuttings, reintroducing them into the flow. The lobes 36 on reduced differential sticking drilling collar 22 help keep cuttings agitated and suspended in the mud. This will help to avoid an accumulation of stationary particles and will prevent the equilibrium layer forming around reduced differential sticking drilling collar 22.

Where reduced differential sticking drilling collar 22 makes contact on all sides, the resulting annular areas act individually. Cuttings within these discrete passageways flow independently. This helps to retain cuttings in suspension and further prevents an equilibrium layer forming.

Collar Diameter

Radial clearance between the collar outer diameter and the wellbore inner diameter may allow for unconstrained drill string motion. Side motion of the drill string may cause vibration and additional forces which can damage equipment or electronics in the BHA. Maintaining concentricity by reducing the distance between the drill collar outer diameter and the wellbore inner diameter is therefore advantageous. Reducing this distance gives less time for the collar to accelerate and contact forces are proportionately reduced. Kinetic energy on impact increases with the velocity squared.

When using a cylindrical prior art drilling collar 22′ the outer diameter of cylindrical prior art drilling collar 22′ is the only variable with which to adjust flow area. In contrast, reduced differential sticking drilling collar 22 has a fluted profile. This allows design flexibility to adjust and optimize the outer profile. For example, the outer diameter of reduced differential sticking drilling collar 22 may be increased while simultaneously increasing the depths of the lobes to compensate and maintain an optimum flow area.

The outer diameter of reduced differential sticking drilling collar 22 can be increased above that of cylindrical prior art drilling collar 22′ while still allowing sufficient annular flow area. A reduced differential sticking drilling collar 22 may be fully stabilized. As used in this disclosure, a fully stabilized drilling collar makes contact at multiple locations around the circumference of the drilling collar such that lateral motion is predominantly constrained. In such embodiments the outer diameter of the drilling collar is substantially similar to the hole diameter.

Contact forces against cylindrical prior art drilling collar 22′ occur at one side. Reaction forces at the contact area are all biased in one direction. As a result, lateral vibration may be induced, which can be damaging to connections and components. A fully stabilized reduced differential sticking drilling collar 22 will see forces distributed over multiple lobes around the collar perimeter. This may make reduced differential sticking drilling collar 22 less susceptible to lateral vibration. Further, reduced differential sticking drilling collar 22 is spiralled so rotation continually changes vector directions and longitudinal position of these contact points.

A comparison of the radial clearance for both cylindrical prior art drilling collar 22′ and reduced differential sticking drilling collar 22 is tabulated in Table 7.

TABLE 7 Maximum Radial Clearance Maximum Collar Outer Open Hole Radial Radial Diameter Diameter Clearance Clearance Collar Shape (in) (in) (in) Reduction Cylindrical Prior Art 6.75 8.5 1.75 Drilling Collar Reduced Differential 8.4 8.5 0.1 94% Sticking Drilling Collar Cylindrical Prior Art 6.75 9. 2.25 Drilling Collar Reduced Differential 8.4 9.0 0.6 73% Sticking Drilling Collar Cylindrical Prior Art 6.75 9.5 2.75 Drilling Collar Reduced Differential 8.4 9.5 1.1 60% Sticking Drilling Collar

Weight

Drilling collars are used to increase the weight on bit. Cylindrical prior art drilling collar 22′ can only be optimized by a single dimension, the outer diameter. In contrast, reduced differential sticking drilling collar 22 has many variables which affect collar weight, including: outer diameter, number, size and depth of flutes, helix pitch, or edge radii, as an example. The number of variables for the reduced differential sticking drilling collar 22 provides more options compared to prior art drilling collars for optimizing the reduced differential sticking drilling collar 22, which can be better tailored to particular applications.

The geometry of reduced differential sticking drilling collar 22 significantly increases collar weight over cylindrical prior art drilling collar 22′, as shown in Table 8.

TABLE 8 Collar Weights Collar Outer Collar Shape Diameter (in) Weight (lbf/ft) Weight Increase Cylindrical Prior 6.75 105.08 — Art Drilling Collar Reduced 8.4 130.32 24% Differential Sticking Drilling Collar

Because prior art fluted drilling collars are essentially prior art cylindrical drilling collars with a groove carved into the outer diameter, prior art fluted drilling collars will have a reduced weight compared to an equivalent prior art cylindrical drilling collar. As an example, a prior art fluted drilling collar can weigh 96% of an equivalent prior art cylindrical drilling collar.

Bending Stiffness

Geometry affects bending stiffness, with the bending stiffness proportionate to the second moment of area. Stiffness is increased when material is placed further from the neutral axis.

Reduced differential sticking drilling collar 22 allows for a large diameter while also maintaining adequate annular flow area. Reduced differential sticking drilling collar 22 can therefore be stiffer than cylindrical prior art drilling collar 22′.

Collar stiffness (which is proportionate to Second Moment of Area) is reported in Table 9.

TABLE 9 Collar Stiffness (Second Moment of Area) Collar Outer Second Moment of Benefit Collar Shape Diameter (in) Area (in⁴) (%) Cylindrical Prior 6.75 99.9 — Art Drilling Collar Reduced 8.4 121.5 21.5% Differential Sticking Drilling Collar

Because prior art fluted drilling collars are essentially prior art cylindrical drilling collars with a groove carved into the outer diameter, prior art fluted drilling collars will have a stiffness is marginally less than a cylindrical collar.

Embodiments of this disclosure therefore provide a drilling collar having deep spiral troughs and an outer diameter that is close in size to the inner diameter of the wellbore. This allows for a reduction in surface area in a single plane and a more balanced distribution of forces. The larger outer diameter provides for a stiffer drilling collar with self-stabilization that will allow for a larger weight to be applied on the drilling bit without deforming the drilling collar. This results in faster rates of penetration, a better quality wellbore, and improved directional stability. When rotated the flutes provide a continuously changing but stable support for the collar. The areas in contact with the formation change as the collar is rotated. When a porous formation is encountered a traditional cylindrical smooth collar would be forced against the formation and when rotated the contact area and will remain the same, irrespective of the collar rotating. In contrast, the low stick collar contact point moves on rotation. This helps the collar break free when differentially stuck.

Embodiments of this disclosure, therefore, are well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others that are inherent. While embodiments of the disclosure has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present disclosure and the scope of the appended claims. 

What is claimed is:
 1. A system for drilling a subterranean well, the system including: a reduced differential sticking drilling collar, the reduced differential sticking drilling collar being an elongated tubular member having: a minimum outer diameter at an uphole end and at a downhole end; a collar body located between the uphole end and the downhole end, the collar body having a cross section of alternating lobes and troughs, the collar body having a maximum outer diameter that is larger than the minimum outer diameter; and a taper section extending from the minimum outer diameter to the maximum outer diameter; where the lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern.
 2. The system of claim 1, where the taper section includes reaming elements, the reaming elements extending radially outward from the taper section.
 3. The system of claim 1, where the taper section includes cutting elements, the cutting elements extending obliquely radially outward from the taper section.
 4. The system of claim 1, where an angle of the taper section relative to a central axis of the reduced differential sticking drilling collar is in a range of 10 to 30 degrees.
 5. The system of claim 1, where a number of lobes of a cross section of the collar body is at least three.
 6. The system of claim 1, where the lobes of the collar body wind around no more than one half of a circumference of the collar body over an entire axial length of the lobes.
 7. The system of claim 1, where the lobes of the collar body and the troughs of the collar body are curved in shape.
 8. The system of claim 1, where an outermost surface of the lobes is parallel to a central axis of the reduced differential sticking drilling collar.
 9. A system for drilling a subterranean well, the system including: a rotating drilling string formed of a plurality of tubular drill string joints, each of the plurality of tubular drill string joints having a joint outer diameter, a reduced differential sticking drilling collar secured in-line with the plurality of tubular drill string joints, the reduced differential sticking drilling collar having: a minimum outer diameter at an uphole end and at a downhole end; an uphole connector operable to secure the uphole end to an uphole adjacent drill string member; a downhole connector operable to secure the downhole end to a downhole adjacent drill string member; and a collar body located between the uphole end and the downhole end, the collar body having a cross section of alternating lobes and troughs, the collar body having a maximum outer diameter that is larger than the minimum outer diameter and larger than the joint outer diameter; where the lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern.
 10. The system of claim 9, further including a taper section extending from the minimum outer diameter to the maximum outer diameter, where an angle of the taper section relative to a central axis of the reduced differential sticking drilling collar is in a range of 10 to 30 degrees.
 11. The system of claim 9, where the lobes of the collar body wind around no more than one half of a circumference of the collar body over an entire axial length of the lobes.
 12. A method for drilling a subterranean well, the method including: providing a reduced differential sticking drilling collar, the reduced differential sticking drilling collar being an elongated tubular member having: a minimum outer diameter at an uphole end and at a downhole end; a collar body located between the uphole end and the downhole end, the collar body having a cross section of alternating lobes and troughs, the collar body having a maximum outer diameter that is larger than the minimum outer diameter; and a taper section extending from the minimum outer diameter to the maximum outer diameter; where the lobes of the collar body wind along an entire axial length of the collar body in a spiral pattern; securing the reduced differential sticking drilling collar in line with a plurality of tubular drill string joints to form a drilling string; and rotating the drilling string to form the subterranean well.
 13. The method of claim 12, where the taper section includes reaming elements extending radially outward from the taper section, and the method further includes reaming within a wellbore of the subterranean well with the reaming elements.
 14. The method of claim 12, where the taper section includes cutting elements extending obliquely radially outward from the taper section, and the method further includes cutting within a wellbore of the subterranean well with the cutting elements.
 15. The method of claim 12, where an angle of the taper section relative to a central axis of the reduced differential sticking drilling collar is in a range of 10 to 30 degrees.
 16. The method of claim 12, where a number of lobes of a cross section of the collar body is at least three.
 17. The method of claim 12, where the lobes of the collar body wind around no more than one half of a circumference of the collar body over an entire axial length of the lobes.
 18. The method of claim 12, where the lobes of the collar body and the troughs of the collar body are curved in shape.
 19. The method of claim 12, where an outermost surface of the lobes is parallel to a central axis of the reduced differential sticking drilling collar. 